Evidence suggests that HIV persistence may result from its ability to irreversibly damage the acquired immune system shortly after infection is established. CD4+CD25+ T regulatory (Treg) cells with T cell immunosuppressive properties are possible mediators of this early T cell immune dysfunction. CD4+CD25+ Treg cells harbor a productive virus infection in HIV infected people and FIV infected cats, and are activated for potent immunosuppressor function in vivo. While CD4+CD25+ Treg cells have been shown to play a role in regulating CD4+ and CD8+ immune responses against HIV and FIV antigens in vitro, it is not known at what time following infection these cells become infected and activated or what effect they might have on the early protective T cell immune response. To determine the role of Treg cells in the immunopathogenesis of HIV infection we propose to utilize the FIV model to test our hypothesis that CD4+CD25+ Treg cells become infected and activated during the acute stage of FIV infection leading to the immunosuppression of CD4+ T helper (Th) cell anti-FIV responses. Cats will be experimentally infected with the NCSU-1 isolate of FIV and blood and lymph node biopsies will be collected at 1 week intervals following inoculation. Real-Time PCR and anti-FIV-gag intracellular staining will be used to determine the time of infection, number of copies of the virus and absolute numbers of CD4+CD25+ and CD4+CD25- infected T cells. These data will be correlated with plasma viremia levels as detected by RT-PCR. Flow cytometry and cell sorting will assess the expression of phenotypic activation markers (CD80, CD86 and CTLA4) and markers for regulatory function (FoxP3, TGF?1) in infected and non-infected CD4+CD25+ and CD4+CD25- T cell subsets at different time points during the acute phase of infection. In addition, experiments will be designed to assess changes in the suppressive function of CD4+CD25+ Treg cells on target CD4+ Th cell immune responses, such as IL-2 and IFN-y production and cell proliferation. In vitro infection of CD4+CD25- and CD4+CD25+ T cell populations will be used to corroborate the results obtained from in vivo studies. HIV is currently estimated to infect more than 37 million people worldwide. FIV is a well established model of HIV/AIDS. The results of these experiments will have implications in future therapeutic strategies against HIV and FIV, and contribute to our current understanding of Treg cell function.